This invention relates to a method for printing substrates comprising a print enhancing fluid.
Applying images to substrates by utilizing pigment or dye based ink compositions is well known in the art. These images are generally applied for the purpose of making the article more aesthetically pleasing to the consumer.
One of the difficulties historically experienced with printed substrates (for example absorbent disposable paper products such as facial tissue, bath tissue, table napkins, wipes, diapers, woven disposable fabrics, nonwovens, wovens, cotton pads, and the like) that are printed with pigment based ink compositions is the tendency for the ink to rub-off of the surface of the paper upon exposure of the paper to liquids. The problem is even more pronounced for those absorbent disposable paper products printed with inks exhibiting relatively high color densities.
The tendency for the ink to rub-off of the printed paper product increases as the printed paper is exposed to liquids such as tap water. Furthermore, exposing the printed paper to common household cleaning products containing solventized alkaline liquids, or acid-containing cleaning liquids tends to increase ink rub-off as compared to exposure of the paper to tap water alone.
Commonly assigned U.S. Pat. No. 6,096,412 issued to McFarland et al. on Aug. 1, 2000, teaches an absorbent disposable paper product printed with inks which exhibit resistance to rub-off.
One of the drawbacks associated with using rub resistant inks relates to printing press hygiene. Inks which adhere well to the substrate often exhibit similar properties when in contact with the printing press. In particular, the print plates tend to accumulate ink deposits which can eventually lead to print defects in the printed substrate. In order to prevent print defects more frequent cleaning of the printing press is necessitated. This can lead to reduced printing process efficiency and increased cost associated with the installation and maintenance of printing press cleaning equipment.
Another drawback relating to printing substrates with pigment based ink compositions is the cost of the ink. The cost of the ink represents a substantial raw material cost in relation to the production of the printed paper products. A significant portion of the cost of the ink is due to the pigment concentration of the ink. For example, in order to produce printed paper products which exhibit high color density print images, a high concentration of ink pigment is required (i.e.; the color density of the print image is proportional to the concentration of ink pigment utilized to print the image). Therefore, all else being equal, a higher concentration of ink pigment yields a higher print density, but at a higher cost. The cost becomes an especially relevant factor when printing on highly absorbent paper products.
Yet further, when printing halftone dots with ink, one way to vary color density is by varying the size of the halftone dots. During the printing process, as the halftone dot is applied to the substrate with the ink an increase in halftone dot diameter on the substrate is typically observed. This is as a result of the wet ink spreading on the substrate. This increase in halftone dot diameter is referred to as dot gain.
Dot gain is one factor which impacts the color density of the printed substrate""s image area. Historically, dot gain has been viewed as a drawback of halftone dot printing as it tends to degrade the fine detail within the image area. Furthermore, dot gain does not allow for selective ink spreading on the substrate. Yet further, it does not provide variable color density in a given printed area. Because of these drawbacks, efforts have been made through the years to devise printing techniques which minimize dot gain.
It would be desirable to produce a printed substrate having higher color density images without the need to use more concentrated ink. It would also be desirable to produce printed substrates such as printed paper products without the need to use rub-resistant ink compositions. Furthermore, it would be desirable to vary color density in a given print region of the-substrate by controlling the spreading of wet ink in this region.
It is surprising to find that the print enhancing fluid of the present invention when applied to a substrate prior to the ink enhances the color density of the printed substrate image area without requiring the use of more concentrated ink or rub-resistant ink compositions. Furthermore, it allows color density in a given print region to be varied by controlling the spreading of the wet ink on the substrate.
The benefit of this invention is the ability to provide substrates such as printed paper products having print images which exhibit rub resistance and higher color densities without the need for more concentrated ink or rub-resistant ink compositions. As the present invention is capable of providing print images which exhibit rub resistance and higher color densities without the need for more concentrated ink formulations or rub-resistant ink compositions, the raw material cost to produce the products of the present invention is lower. This invention also provides a method for printing variable color densities within the same print region. Yet further the invention has broad applicability to a range of printing inks, substrates, and printing processes.
This invention relates to a method for printing indicia on a substrate. The substrate has a first outer surface and a second outer surface. A print enhancing fluid is applied to at least one of the first outer surface and the second outer surface of the substrate. An indicia comprised of ink is then applied to at least one of the first outer surface and the second outer surface of the substrate.
The print enhancing fluid is miscible with the ink. It can be a polar or non-polar fluid. It can be hydrophilic or hydrophobic. It can be in the form of a solution or emulsion.
It can be applied to the substrate by any printing method including but not limited to ink jet, silk screen, rotogravure, letterpress, intaglio, lithography, and flexography.